The large-scale adoption of consumer mobile computing devices such as smartphones offers new possibilities for human interaction with computers and as well as the surrounding environment. Next generation mobile devices are expected to provide information by displaying it in a different manner than the current hand portable display screen. Advances in projection display technologies are enabling near the eye displays, such as a pair of see through glasses with overlaid information displayed directly to the user.
See-through displays have been used for decades for defense applications. For example, Jet fighter pilots have been using heads-up displays (HUDs) in aircraft and helmet-mounted displays (HMDs) to provide navigational and other critical information to the pilot in his/her field of view. While projection technology is advancing, there is still currently a difficult trade-off between field of view and the bulk and weight in see-through HWDs. In most cases a significant field of view (>30-40 degrees) requires bulky optics making their usage difficult for many applications. Smaller field of view systems have been introduced with more acceptable form-factors, but the challenge remains to create useful implementations of see-through displays with aesthetically pleasing form factors for a wide range of applications and even everyday use.
One primary challenge in the design of HWDs is the expansion of the so-called eyebox of the display. The eyebox is an optical system tolerance to the placement and movement of the wearer's eye. This corresponds closely to the exit pupil of the optical system. The conventional approach in HWDs is to expand the optical system's exit pupil by various means. However this usually leads to a more bulky optical system.
HWDs are often implemented using microdisplay panels, such as LCOS and OLED panel arrays, which are presented to the eye in a pupil forming or non-pupil forming arrangement of imaging optics which allow the wearer to see a distant image of the microdisplay. Another but less common approach is retinal projection. Retinal projection uses a scanning element to raster scan an image directly onto the user's retina. Retinal projection displays originate with the scanning laser ophthalmoscope (SLO) developed in 1980. The technology was later developed into the virtual retinal display, led by Tom Furness at the University of Washington's HITLab in the 90s (Thomas A. Furness et al. “Display system for a head mounted viewing transparency” U.S. Pat. No. 5,162,828, filed 1989), (Thomas A. Furness et al. “Virtual retinal display” U.S. Pat. No. 5,467,104, filed 1992). Since then many HWD patents have been filed using MEMS based scanning projectors, i.e. retinal displays. Of particular note are patents owned by the University of Washington and Microvision (a spinoff of University of Washington) who led early efforts to commercialize the virtual retinal display in the mid-late 90s. The majority of this work involved efforts to expand the exit pupil of the system, which is otherwise small due to the low étendue laser source. The prevalent method found in patent literature is the use of a diffractive or diffusing screen to expand the beam, which is then re-collimated before presenting it to the eye (Joel S. Kollin et al, “Virtual retinal display with expanded exit pupil” U.S. Pat. No. 5,701,132, filed 1996). The drawback of this approach is that the beam expansion optics creates added optical bulk with trade-offs similar to other conventional HWD approaches.
There have been methods to create multiple and/or steerable small exit pupils. These methods have used an array of lasers for multiple eyebox locations in conjunction with eye-tracking (M. Tidwell, “Virtual retinal display with scanner array for generating multiple exit pupils”, U.S. Pat. No. 6,043,799, filed 1998), (M. Tidwell, “Scanned retinal display with exit pupil selected based on viewer's eye position,” U.S. Pat. No. 6,204,829, filed 2000). Systems with steerable exit pupils based on eye-tracking have also been proposed (John R. Lewis et al., “Personal display with vision tracking” U.S. Pat. No. 6,396,461, filed 1998). These systems relied on eye tracking and did not use a method to unify the images produced by the multiple exit pupils.
There have been several HWD implementations using of Holographic Optical Elements (HOEs). Takahashi et al. have applied for a patent for a system using an HOE and Maxwellian view arrangement (Hideya Takahashi et al., “Image Display Unit and Electronic Glasses”, U.S. patent application Ser. No. 11/576,830, filed 2005), however the system in this patent does not appear to consider a laser scanning projector, but rather an expanded beam passed through a spatial light modulator. In addition there is no discussion of multiplexing or multiple exit pupils.
The concept of using a microdisplay in conjunction with a single layer hologram as a beam combiner is also known prior art—for example U.S. Pat. No. 3,940,204. From a related journal publication on the work, it was described that aberrations were quite large in these systems. This was largely due to the requirement for a large exit pupil of 12-15 mm at the eye. This creates a larger aperture, “faster” optical system, which is more difficult to control in terms of aberrations. The size of the projector is also directly proportional to the numerical aperture of the beam and the size of the exit pupil at the eye location.